Constrained geometry catalysts (CGCs) have received considerable attention because of their ability to copolymerize ethylene with α-olefins, forming linear low density polyethylene (LLDPE) on the commercial scale of several billion pounds per year (see Stevens et al., Eur. Patent Appl. EP 416815-A2, 1991 (Dow Chem. Co); Canich, Eur. Patent Appl. EP 420436-A1, 1991 (Exxon Chem. Co.); Shapiro et al., Organometallics, vol. 9, pp. 867–869, 1990; Shapiro et al., J. Am. Chem. Soc., vol. 116, pp. 4623–4640, 1994; McKnight et al., Chem. Rev., vol. 98, pp. 2587–2598, 1998; McKnight et al., Organometallics, vol. 16, pp. 2879–2885, 1997; Lanza et al., Organometallics, vol. 21, pp. 5594–5612, 2002; and Xu et al., Macromolecules, vol. 34, pp. 2040–2047, 2001). Reportedly, α-olefin incorporation exceeds that with other organometallic catalyst systems because of the steric accessibility afforded by the linked cyclopentadienyl-amido ancillary ligand. Although the precatalyst Me2Si(η5-C5Me4)(η1-N-tBu)TiMe2 is considered to be the commercial standard for ethylene/1-octene copolymerizations (Boussie et al., J. Am. Chem. Soc., vol. 125, pp. 4306–4317, 2003), it has been shown that indenyl-amido CGC systems such as Me2Si(η5- 2-Me-benz[e]Ind)(η1-N-tBu)TiCl2/MAO (MAO=methylaluminoxane) can induce high activity and moderate α-olefin incorporation in ethylene/1-octene copolymerizations (see Xu et al., Macromolecules, vol. 31, pp. 4724–4729, 1998). In contrast, existing fluorenyl-amido polymerization systems such as Me2Si(η5-C13H8)(η1-N-tBu)TiCl2/MAO are typically inferior with respect to activity, comonomer incorporation, molecular weight, thermal sensitivity, and catalytic lifetime. See Xu et al., Macromolecules, vol. 34, pp. 2040–2047, 2001; Okuda et al., Organometallics, vol. 14, pp. 789–795, 1995; Dias et al., J. Organomet. Chem., vol. 508, pp. 91–99, 1996; Xu et al., Macromolecules, vol. 31, pp. 2395–2402, 1998; Alt et al., J. Organomet. Chem., vol. 572, pp. 21–30, 1999; and Razavi et al, J. Organomet. Chem., vol. 621, pp. 267–276, 2001.
In 1988, Ewen, Razavi, et al. first reported the use of a single-site catalyst for the preparation of syndiotactic polypropylene (s-PP), a high-melting, crystalline thermoplastic. The ansa-zirconocene Me2C(η5-C5H4)(η5-C13H8)ZrCl2, upon activation with methylaluminoxane (MAO), produced stereoregular s-PP with a Tm (melting temperature) of 138° C. and a stereochemical [rrrr] pentad fraction of 86% (Ewen et al., J. Am. Chem. Soc., vol. 110, pp. 6255–6256; For a general review of propylene polymerization via metallocene catalysts see: Resconi et al., Chem. Rev., vol. 100, pp. 1253–1345, 2000). Since this report, commercial interest in s-PP has increased; global production is anticipated to grow to 4×108 kg of s-PP per year by 2010, approaching 1% of worldwide polypropylene capacity (Shamshoum, E.; Schardl, J. in Metallocene-Catalyzed Polymers—Materials, Properties, Processing and Markets; Benedikt, G. M., Goodall, B. L., Eds.; Plastics Design Library: Norwich, N.Y., 1998, pp. 359–368.). Despite a number of superior physical properties—such as greater optical clarity, tear resistance, and impact strength—the development of s-PP has lagged behind that of isotactic polypropylene (i-PP), which can be made with [mmmm]>99% and a Tm as high as 166° C. (Ewen et al., J. Am. Chem. Soc., vol. 123, pp. 4763–4773, 2001).
Attempts to improve catalyst activity and syndioselectivity have resulted in several second generation single-site catalysts, including doubly-bridged metallocenes (e.g., [(1,2-Me2Si)2 {η5-C5H3} {η5-C5H-3,5-(CHMe2)2}]ZrCl2, Veghini et al., J. Am. Chem. Soc., vol. 121, pp. 564–573, 1999), fluorenyl-amido constrained geometry catalysts (CGCs) (e.g., Me2Si(η1-N-tBu)(η5-3,6-tBu2C13H6)ZrCl2, Razavi et al., J. Organomet. Chem., vol. 621, pp. 267–276, 2001; Busico et al., Macromol. Chem. Phys., vol. 204, pp. 1269–1274), coordination compounds (e.g., bis[N-(3-(SiMe3)salicylidene)-2,3,4,5,6-pentafluoroanilinato]-TiCl2, Mitani et al., J. Am. Chem. Soc., vol. 124, pp. 7888–7889, 2002), and sterically expanded versions of the parent Ewen-type catalyst such as Me2C(η5-C5H4)(η5-C29H36)ZrCl2, which provided markedly improved s-PP with [rrrr]=92% and an unannealed Tm of 154° C. (Miller et al., Organometallics, vol. 23, pp. 1777–1789, 2004). These efforts, however, have generally suffered from arduous catalyst syntheses (e.g., [(1,2-Me2Si)2{η5-C5H3} {η5-C5H-3,5-(CHMe2)2}]ZrCl2, Veghini et al., J. Am. Chem. Soc. 1999, 121, 564–573), low catalytic activity (e.g., bis[N-(3-(SiMe3)salicylidene)-2,3,4,5,6-pentafluoroanilinato]-TiCl2, Mitani et al., J. Am. Chem. Soc., vol. 124, pp. 7888–7889), thermal instability, and poor syndioselectivity (e.g., Me2Si(η1-N-tBu) (η5-3,6-tBu2C13H6)ZrCl2), Razavi et al., J. Organomet. Chem., vol. 621, pp. 267–276, 2001; Busico et al., Macromol. Chem. Phys., vol. 204, pp. 1269–1274, 2003). Accordingly, a catalyst or catalyst system capable of overcoming one or more of these limitations would be exceedingly beneficial.